Corals as Environmental Indicators

Study of annual banding characteristics of corals from various locations on the Bermuda reefs has revealed the following: Annual patterns of growth are similar between individual coral heads within the same general environment. Between environment correlation is present but less distinct. Growth rate is highest in Castle Harbor and the north lagoon; variability in growth is highest in Castle Harbor, intermediate on open reefs, and lowest in the north lagoon. The applicability and results of comparing annual band series with recorded environmental variables are discussed. Evidence based on coral age distributions from Bayesian statistical analysis and on variations in annual banding characteristics suggests catastrophic mortality of corals in Castle Harbor as a result of a dredging operation between 1941-1943. We demonstrate 1) most living corals in the harbor today originated after dredging, 2) living harbor corals older than 45 years are virtually absent while corals external to the harbor range to ages of at least 300 years, and 3) banding patterns of individuals in a collection of Castle Harbor dead corals are generally similar. These results suggest a common death event which most logically is postulated to be dredging. Calculations of energy requirements necessary for self-cleaning indicates that expectation of survival through an event of increased sedimentation becomes small for large corals. This conclusion is in accord with our field observations that corals older than 10-12 years at dredging have negligible survivorship, while corals and their included banding, younger than 12 years, are little influenced by the event.